This invention relates to the provision of a phase changing effect upon the propagation of guided light. (For the purposes of this specification, the terms xe2x80x98opticalxe2x80x99 and xe2x80x98lightxe2x80x99 should be understood as pertaining not only to the visible part of the electromagnetic spectrum, but also to the infra-red and ultra-violet parts that bound the visible part.)
In certain optical communications applications involving the transmission of data over waveguides there is a requirement to effect a change of the phase of propagating light.
A controllable change of phase of light may, for example, be employed in situations in which light launched into one waveguide is caused to interfere optically with that propagating in another waveguide, such as for instance in an optical waveguide Mach Zehnder interferometer used as an optical attenuator or spectrally selective filter.
A Mach Zehnder interferometer of this type is described for instance with particular reference to FIG. 17 of a review paper by Masao Kawachi entitled, xe2x80x98Silica waveguides on silicon and their application to integrated-optic componentsxe2x80x99, Optical and Quantum Electronics 22 (1990) pp 391-416. This is an example of an interferometer in which phase control is effected using a Joule heater to change the optical path length of one of the interference arms by making use of the fact that the effective refractive index of an integrated optics waveguide exhibits a temperature coefficient (dneff/dT). The sensitivity of such a phase controller is limited by the fact that the thermo-optic coefficient of silica is relatively small (dn/dT xcx9c1xc3x9710xe2x88x925, where T is measured in xc2x0 C.). Many other materials exhibit significantly larger thermo-optic coefficients, but are difficult to form into low-loss single mode waveguides, and so are not well suited as materials from which to construct an acceptable optical waveguide exhibiting a phase sensitivity large compared with that of a silica waveguide based thermo-optic phase controller.
An object of the present invention is to provide an optical waveguide phase adjuster of increased sensitivity.
According to a first aspect of the present invention there is provided a monolithic length of optical waveguide divided into a plurality of concatenated waveguide sections by a set of transverse slots, each occupied by a non-waveguiding controllable refractive index element, each slot having a linear dimension, in the direction of propagation of light in the waveguide, that provides, between the two waveguide sections that it separates, a coupling loss not exceeding 0.3 dB.
According to a second aspect of the present invention there is provided a monolithic length of optical waveguide divided into a plurality of concatenated waveguide sections by a set of transverse slots, each occupied by a non-waveguiding controllable refractive index element, each slot having a linear dimension, in the direction of propagation of light in the waveguide, not exceeding 25 xcexcm.
The invention makes use of the fact that, for small separations between the ends of identical waveguides with co-aligned axes, the optical coupling loss is quite small. However, as the separation increases, so the coupling loss begins to increase in a manner which resembles an exponential increase inasmuch as the rate of increase increases with increasing separation. In many instances the optical sensitivity of the medium occupying the gap between the adjacent ends of these two waveguides is so small that the obtaining of the requisite range of phase adjustment in this medium would require a gap thickness much too great to provide an acceptably low coupling loss between the two fibres. By reducing the magnitude of the gap by a factor xe2x80x98nxe2x80x99, while at the same time arranging to have a concatenation of waveguide sections defining a set of xe2x80x98nxe2x80x99 of the smaller gaps optically in series, matters can be arranged to provide an aggregate range of phase adjustment comparable with that of the single large gap. The coupling loss of one of these smaller gaps is much more than n-times smaller than that of the large gap, while the aggregate coupling loss of the series combination of all the smaller gaps is approximately n-times larger than that of a single small gap. Therefore the aggregate coupling loss of the series combination of all the xe2x80x98nxe2x80x99 smaller gaps is much less than the coupling loss of the single large gap, and hence, with the appropriate choice of the factor xe2x80x98nxe2x80x99, the aggregate coupling loss can often be made small enough to be acceptable.
In some circumstances the optical sensitivity of the intervening medium can be so great that a single gap is sufficient on its own to provide the required range of phase adjustment.
According to a third aspect of the present invention there is provided a monolithic length of optical waveguide divided into a pair of concatenated waveguide sections by a single transverse slot occupied by a non-waveguiding controllable refractive index element, the slot having a linear dimension, in the direction of propagation of light in the waveguide, that provides, between the two waveguide sections that it separates, a coupling loss not exceeding 0.3 dB.
According to a fourth aspect of the present invention there is provided a monolithic length of optical waveguide divided into a pair of concatenated waveguide sections by a single transverse slot occupied by a non-waveguiding controllable refractive index element, the slot having a linear dimension, in the direction of propagation of light in the waveguide, not exceeding 25 xcexcm.
Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, from the drawings and from the claims.